1. Field of the Invention
This invention relates to computer graphics systems and, more particularly, to methods and apparatus for storing signals appearing in both interlaced video and non-interlaced graphics modes in a single frame buffer for presentation on either an interlaced or non-interlaced output display device.
2. History of the Prior Art
It is the vision of many that in the near future a person sitting at a personal computer will be able to call information from a number of different sources. For example, it is expected that a person will be able to hear telephone and radio communications, view television or recorded motion pictures, play stereo recordings of music, and operate computer graphical and text programs. It is also expected that all of these operations will be possible at the same time so that, for example, a television program may appear in one window of a computer display while a computer graphics program is running in another window or computer graphics material may appear as an overlay on the television program.
It is much easier to visualize the results that one would like to reach than to reach those results, especially where the results require the combining of television (video) signals with computer graphics signals on the same output monitor. The crux of the problem is that, although both types of signals are electrical, they arrive in entirely different formats for their two purposes. The television signals are analog and must first be converted to digital representations for presentation on a computer monitor. Moreover, the television signals (video) are presented at a different frequency in an interlaced pattern consisting of a first field which may be approximately 240 active lines followed by a second field of approximately 240 active lines about one-sixtieth of a second later to form a complete picture. This allows a less expensive monitor to present pictures which are entirely acceptable for television. However, such a monitor is not acceptable for computer graphics where much more detail must be displayed and manipulated. Consequently, a typical computer monitor may display 480 lines of data in a non-interlaced mode.
Thus, the data from these two different sources of two different types, interlaced and non-interlaced, must somehow be presented in a form which can be handled by a personal computer. The usual method suggested is to convert the video data to digital data and place it in a first frame buffer, place the computer data in a second frame buffer, and somehow switch between the two frame buffers in presenting the data to an output monitor.
However, the video data stored in the frame buffer is still in interlaced form if it came from a television or similar source while the computer data is stored in non-interlaced form. The visionary also expects to be able to present the output on either an interlaced television type monitor or a computer monitor of some sort. Thus, interlaced video data and non-interlaced computer data must somehow be intermingled and displayed on both interlaced and non-interlaced monitors at the option of the operator.
Presenting interlaced data on a monitor designed to display interlaced signals is not a problem for such a monitor simply takes the 240 lines of interlaced information available in a first frame and presents it on the 240 lines available on the monitor. Then it follows this with the next 240 lines which are interleaved between and offset in time from the first 240 lines to make up the complete picture.
However, presenting the non-interlaced data on a monitor designed to display interlaced signals is a greater problem. Non-interlaced data has 480 lines which are not offset in time. If every other line is displayed to make up a first frame and then the alternate 240 lines are displayed to make up an interleaved second frame, the fact that the lines are not offset in time causes flickering which is disconcerting to the viewer. Consequently, the lines of the non-interlaced computer display must somehow be adapted to appear correct to the viewer when presented on an interlaced output monitor.
In a similar manner, presenting non-interlaced data on a monitor designed to display non-interlaced signals is not a problem for such a monitor simply takes the 480 lines of non-interlaced information available and presents it all on the monitor. However, presenting the interlaced data on a monitor designed to display non-interlaced signals is a greater problem. Interlaced data has only 240 lines per field followed by a second 240 lines which are offset in time. If the 240 lines from both fields are displayed together to make up a non-interlaced frame, the fact that lines which are time offset are presented together provides a picture which is incorrect when motion occurs. Consequently, the lines of the interlaced video display must somehow be adapted to appear correct to the viewer when presented on an non-interlaced output monitor.
Thus, it is clear that whether the monitor handles either interlaced or non-interlaced data, you must somehow change some of the data if both types are to be displayed on the same monitor.
An additional problem arises because of the nature of systems which are able to present information from a plurality of sources at the same time. Most computer systems accomplish this by means of windows, regions on a computer output display in each of which information from different programs may be presented. These windows may be moved about on the screen and retain the same information whatever position they are in. The information in any window may be manipulated apart from the other information on the output display.
It is desirable that this windowing ability be retained since the primary function of a computer is to deal with computer programs. Consequently, a video display should appear in a window. Although windows are usually rectangular, when a window is in the background on a computer display and is overlaid with other windows, the window is no longer a rectangle but an arbitrary shape. It is thus desirable that a video image be able to appear in such an arbitrarily shaped region on a computer output display. Moreover, it is desirable that a system be capable of intermixing computer graphics images with the video graphics display so that, for example, graphics or text material may overwrite the video images in the video window. Consequently, it is desirable that video images be placed in a completely arbitrarily shaped window on a computer output display.